Part Forty Two – Water Treatment

This blog entry is just looking in detail at the ozone treatment part of our borehole water system.

Our borehole water has a high concentration of ferrous iron, this is what’s often called “clear iron” as it doesn’t colour the water. It’s fairly common in water drawn from deeper aquifers, particularly in areas where there are natural iron deposits or soft (often slightly acidic) water that dissolves iron (and other metals, like manganese) from the surrounding rock. In our case our water comes from the Lower Greensand formation, an aquifer that is known to have water with high concentrations of ferrous iron as well as dissolved hydrogen sulphide gas (the “rotten eggs” smelly stuff).

The traditional way to remove ferrous iron, manganese and hydrogen sulphide from water is to just add oxygen to it, usually in the form of air, but it can be by using an oxidising catalyst. The most common oxidising catalyst is manganese dioxide, used in a filtration bed. This works OK, but it does eventually need replacing, or regenerating in some way, as it will lose its ability to oxidise after a time.

For those interested in the chemistry, ferrous iron converts to ferric iron (rust) when it’s exposed to oxygen. Whereas ferrous iron is fairly soluble in water, ferric iron isn’t really at all, so when this forms it deposits orange/brown particles in the water that can be filtered out.

Hydrogen sulphide can also be reduced by oxidation, leaving insoluble sulphur particles in the water that can be filtered out, but because the hydrogen remains dissolved after oxidation it is better vented off to the atmosphere if possible.

A significant issue here in the UK is that not many people have borehole water supplies, and even fewer have borehole supplies where the water has ferrous iron, manganese or hydrogen sulphide present. Historically I think this is probably because these contaminants mainly come from deep boreholes and areas where they are found may have historically used surface water springs or wells before the advent of widespread mains water supplies, so using water like this was avoided if possible.

In the US, where most rural properties use boreholes and wells for water, and “city water” is only available in fairly big towns and cities, necessity has led to a plethora of treatment systems to make all sorts of water potable. I found that there is a great deal of knowledge readily available from US drillers and water treatment equipment suppliers, just because using a private supply is normal there for most of the population. I’m indebted to a couple of Americans who took the time to give me a great deal of advice by email. Without their help I’d have far less understanding of the best way to treat our water.

The problem with all this expertise being in the US, means that the vast majority of the equipment to deal with water treatment is made in the US, too, and it is pretty expensive to import to the UK. There are some UK suppliers of good US-made treatment equipment, but generally they only stock commonly-used stuff. The best supplier by far, and one that I would strongly recommend, both for their customer service and for their technical knowledge, is GAPS Water Treatment in Rochdale, Lancashire. Not only do they supply complete systems for most commonly encountered water treatment needs, but they also stock lots of spare parts. This latter capability saved me a great deal of money when I wanted a bit of kit they didn’t stock, but found I could easily make it using a cheap (around £5) spare part from them.

So how does our system work? The borehole pump is a standard item, a submersible high pressure pump that feeds water from half way down the borehole, via a length of 25mm MDPE pipe, to the water treatment shed at the rear of the house. One problem with our borehole is that if I pump water at a rate over about 15 litres per minute very fine sand gets pulled through the filter media around the borehole liner. If I keep the flow rate below this I get no sand at all, so I had to limit the flow rate filling the two 300 litre, and one 100 litre, pressure storage tanks to less than 15 litres/min. This sand problem is common in some regions of the USA, so an American company has come up with an automatic flow rate limiter (a bit like an industrial version of the things used to limit shower flow rates) called a Dole Valve. This maintains a constant flow rate no matter what the pressure, within limits. I chose one rated at 2.5 US gallons per minute, which is about 9.5 litres/min.

One advantage of fitting the Dole Valve in series with the main water feed, apart from the control of flow rate, it that it generates a pressure drop across it, that can be quite high. Typically our pump line will be at around 10 bar when water is flowing, whereas the pressure tanks run at between 3 bar minimum and 4 bar maximum, so there is always between 6 bar and 7 bar pressure drop across the Dole Valve.

This pressure drop is very useful, as it allows the use of a simple, low flow rate, venturi to be used to suck air, or in our case air and ozone, into the water supply, whenever the borehole pump is running. This is a very common way to aerate water in US private water supply systems, and there are several manufacturers of small venturis, combined venturis with adjustable flow regulator bypass valves or even one venturi with an automatic bypass valve that uses the internal control washer from a Dole Valve, in effect, in the bypass line.

Venturis are interesting, and worth explaining in a bit more detail. Back around 1738 Bernoulli found that if you increased the velocity of a flowing liquid, by reducing the size of hole it was flowing through, then it created a pressure drop (see here for more details: https://en.wikipedia.org/wiki/Bernoulli%27s_principle).

I’m using a tiny venturi (it flows less than 0.5 litres/min) in parallel with the Dole Valve, to both create suction from that pressure drop, which sucks in ozone and air, and to mix the ozone and air with the water, like the fine spray from an aerosol. This ensures that as much as possible of the ozone, in particular, gets dissolved in the water, where it very quickly oxidises anything in it (and kills off bacteria, viruses and cysts, as a side effect). Ozone is a very good sterilising agent, because of its very powerful oxidising properties, but that’s not principally why I’m using it.

Perhaps an explanation of what ozone is might be useful here. Ozone is a form of oxygen, known as an allotrope. Oxygen is fairly reactive (it’s what causes almost all the corrosion we see on anything) but it’s also stable, as long as there are two oxygen atoms joined together. Almost all the oxygen in the air is made up of two oxygen atoms joined together tightly, and is referred to as O2. If you chuck a load of energy into oxygen, then you can force it to split apart and reform with three oxygen atoms joined together, O3, ozone. The snag is that ozone is highly unstable and exceptionally reactive. It desperately wants to return to the stable allotrope, with just two atoms, so constantly want to try and find a way to get rid of the extra oxygen atom, as quickly as it can.

This is what makes ozone very useful for converting water-soluble ferrous iron oxide (FeO) into insoluble ferric iron oxide (FeO3), as ferrous oxide is also fairly unstable and will grab any oxygen it can find to turn into stable ferric oxide. Much the same happens with other soluble metal oxides in the water, like manganese, or anything that is readily oxidised, like bacteria, viruses, cysts etc. The other useful attribute of ozone is that it is so highly reactive that a small amount will oxidise relatively large amounts of water. Ozone is more reactive that chlorine or hydrogen peroxide, and will oxidise just about anything, including sulphides, some plastics and most metals. There is more on the characteristics of ozone here: https://en.wikipedia.org/wiki/Ozone

So, I now had a way of being able to suck air, or ozone, into the supply water, by using a small venturi, but I needed three other bits of kit to make this a practical way to treat water; an ozone generator, a reaction tank where the oxidation could take place and a filtration system to remove the oxides and dead bacteria, etc from the treated water.

The ozone generator

Ozone is easy to make using electricity, every time there is a spark generated ozone is generated (it’s the smell people associate with sparks, or even nearby lightning). Sparks are also dangerous, though, and they erode whatever is sparking (like in an arc welder, for example). Luckily there is a way of making ozone that doesn’t need sparks and that’s by using what’s called a corona discharge. This occurs where you have a high voltage on two metal parts, separated by a insulator. Around the edges of any sharp edge, a corona discharge will occur, which is fairly quiet (just a gentle hissing noise) and most importantly it doesn’t erode the metal electrodes.

Because ozone is so useful for oxidising things and sterilising, ozone generator parts are cheap and easy to buy. They are found in things like the small battery operated fridge deodorisers, where the ozone oxidises the smelly chemicals from things like some cheeses, or fish. They are also found in air fresheners in toilets, for the same reason. But, one of the most common applications (at least in the UK) is for treating the water in fish tanks and ponds. Adding small amounts of ozone will kill bacteria, viruses etc in water in which fish are kept, keeping the water clean and the fish healthy.

Ebay, Banggood etc are a great source of ozone generator parts, but the sellers often (almost always!) misrepresent what they are selling and fail to provide anywhere near enough information to be able to use their parts without some experimentation. Hopefully the experiments I’ve done may avoid the need for others to find things out for themselves. The first thing neeeded is a good quality stainless steel tubular ozone generator unit, together with a matching high voltage power supply. I bought a unit from Banggood, in China. The unit I purchased was the result of having bought several units and tried them, and this is without a doubt the best and easiest to use: http://www.banggood.com/220V-5GH-Water-Disinfection-Treatment-Suite-Ozone-Generator-Quartz-Tube-p-1076913.html?rmmds=buy

If that link doesn’t work any more, look for units that look like this and are rated for 5g/hour of ozone production:

This unit was fitted into a metal box, with a cooling fan (it gets hot in use) and 10mm polyurethane tube was used to connect the ozone to the non-return valve on the venturi injector (this tubing has to be ozone resistant, as does the non-return valve – check that the NRV uses Viton seals).

There is a 12V power supply in the metal box, to run the cooling fan and also to power a timer relay, that comes on for 20 seconds whenever the unit is powered on (it is switched on by the submersible borehole pump pressure switch). The power from this timer relay operates a 12V solenoid valve that blows waste water and sediment from the base of the aeration and ozonation tank, to keep it clean. This short burst at the start of every pump cycle has no effect on the household supply, as there is a non-return valve between the aeration tank and the main pressure vessels, so the pressure vessels carry on supply the household supply for this 20 second cleaning period.

The main problem with running an ozone generator on air is that any moisture in the air both reduces the amount of ozone produced and generates small amounts of nitric acid, from a reaction with the nitrogen in the air. This is not really enough to be harmful, but it’s undesirable, so I decided to make up an air drying unit to feed the ozone generator with dry air. This significantly improves the performance of the generator and will make it more reliable and less likely to suffer internal condensation (a problem I had with an early prototype).

The air drier was easy to make, using a standard, fairly cheap, 10” water filter housing, together with a reusable cartridge insert that was intended for use with carbon granules. By filling the cartridge insert with silica gel, I found that I could make a very effective air drier, and by using a clear filter housing, together with colour-indicating silica gel, I can see when to change the drier cartridge and regenerate the old one (regeneration is easy, just put it in an oven set to 100 deg C for a couple hours and it will be ready to use again). I’ve found that a single cartridge lasts in excess of six months, and may well last a lot longer, as the one in use shows no signs of turning blue yet (the colour-indicating beads turn blue when they need regenerating and are orange when they are still OK). This is what a filled filter cartridge insert looks like, these can also be bought on ebay, as can the indicating silica gel beads:

This is fitted into an upside down mounted, clear, water filter housing, like this:

There is one small problem created by the flow resistance of the length of pipe from the venture to the ozone generator (I used 10mm polyurethane, as it’s relatively ozone resistant), the flow resistance of the ozone generator itself and the flow resistance of the air drier unit. Adding an air pump before the air drier was the solution, but after some experiments I discovered that it is vital that the pressure be kept low. If the pressure in the ozone generator chamber exceeds about 0.3 bar, then the corona discharge starts to reduce, and by around 1 bar it has stopped completely. Luckily I managed to find a reliable and low power large aquarium pump that is oil-free and delivers a maximum pressure of 0.18 bar. This is a near-perfect match to the flow and pressure requirements, and delivers just enough pressure at the venturi end to open a sensitive non-return valve (needed to prevent water back flow when the borehole pump and ozone generator system is off). This is what the pump looks like:

Low pop-off pressure non-return valves were used to isolate the pump from the air dryer and also in-line with ozone generator output, in addition to the standard high-pressure NRV on the venturi, to ensure that the air in the pipes and the ozone generator was always kept dry. Alloy NRVs, modified to use silicone rubber seals (also ozone resistant) were used.

Both the pump and the ozone generator are powered from the borehole pump circuit, so they only run when the borehole pump is running, the rest of the time they are turned off.

Venturi injection unit

Next I needed to make up the venturi injection unit. There are commercial units available, ranging from the non-automatic flow control Clack U1202 for around £140, including delivery and tax, like this:

Through to a version with an automatic flow regulator (which is what I needed) for around double the price (it’s $250, but probably ends up being over £300 with shipping and taxes). That seemed an awful lot of money for a bit of machined PVC, and in the photos above I spotted a part I recognised, an injector venturi that’s used in all Clack water softener valves (it’s the white thing secured with a screw in the right hand photo above). I went looking around on the Clack website and found this photo of their range of injector venturis:

This also gave me the part number for the white one!

A quick phone call to GAPS Water Treatment and one was in the post for a few pounds (they keep them all as spares for water softeners). Once it arrived I measured it up, discovered that it flowed about 0.5 litres per minute at the sort of pressure drop I had and that it would create plenty of suction over the whole operating range. All it needed was a housing to hold it and a sensitive non return valve (one that would open with a fraction of a bar of pressure) to fit on the ozone/air inlet side.

I bought a brass ½” BSPF tee (one with three ½” BSPF threaded outlets), a ½” BSPM to ¼” BSPF threaded reducer for the non-return valve, a ½” BSPM brass “iron to iron” connector, a 15mm compression to ½” BSPF adapter and a 15mm to ½” BSPM adapter and I was ready to do. The only part I had to physically make was a bit of 15mm OD brass, machined up to fit inside the tee, with bored holes to suit the diameter of the O seals on the venturi injector. A 6mm hole bored in the side of this allowed ozone/air to reach the suction point. This is a cutaway drawing of the thing, without the non-return valve fitted, that shows how water under pressure comes in at the bottom, with the venturi sucking in ozone and air from the left hand side and the mixed water, ozone and air exiting from the top:

This is what the completed ozone injection side looks like, with all the parts annotated (the 8mm LDPE pipe has been changed for orange 10mm PU pipe and the sensitive non-return valve has been replaced with a stainless steel one with Viton seals since this photo was taken – ozone is definitely corrosive!):

There is a Y screen filter in front of the venturi, with a pressure gauge so I can keep an eye on the inlet pressure (it’s usually around 10 bar when the pump is running and drops to the tank pressure with the pump off). The Dole Valve automatic flow regulating valve is the shiny thing on the left, and most of the water flows through that branch. The ozone/air feed pipe is the white 8mm LDPE pipe (later replaced wit PU pipe) and the highly reactive air/ozone/water mix comes out at the top tee and mixes with the bypass flow.

Reaction or aeration tank

The water needs time to react with the air and ozone, around 30 seconds to 1 minute is enough to fully oxidise anything in the water, and there is also a need to allow the excess gasses to escape, or else the water ends up full of bubbles. After leaving the top of the venturi injection unit, the water/gas mixture flows to a 63 litre capacity reaction, or aeration, pressure vessel. This is a relatively cheap pressure vessel intended to be used as part of a water softener or deionised water system, again available as a spare part, complete with head fitting, from GAPS Water Treatment. I modified the head fitting by drilling and tapping a ¼” BSPF threaded hole in the top, so that I could fit an 8mm dip tube, connected to an automatic air vent. This maintains a small air/ozone pocket at the top, into which incoming air/ozone/water is sprayed, via a grille that is a part of the head fitting. The head fitting has a long rigid pipe, with a strainer at the base, that takes the water outlet from the very bottom of the tank. At the house maximum water demand this tank retains water for around 2 to 3 minutes, more than enough for the complete oxidation reaction to take place, and far longer than is needed for the ozone to kill any bugs. Most of the time the tank will hold water in contact with the air/ozone mix for a lot longer than this, as the water flow rate will rarely exceed about 13 to 15 litres per minute. This is a cross section of the tank (it’s the left hand blue cylinder in the previous photo):

Here’s a photo showing the top end of the tank, with the optically operated pressure switch that controls the borehole pump switch on and switch off pressures:

The water from the bottom of this tank feeds two parallel connected 300 litre accumulators, via a 22mm non-return valve and they then feed water to a backwashing aquamandix and sand filter (the right hand blue cylinder) that has a Clack control valve that backwashes the filter at around 2 am every fourth day, to wash out all the accumulated oxides. Water from the filter feeds two supplies, both with non-return valves. One is an outside tap supply, feeding external taps that also have dual non-return valves, the other supply runs under the slab and up into the house, where it feeds a 100 litre accumulator then a water softener, a 5µ water filter and a UV disinfection unit as a backstop in case anything isn’t killed by the ozone, or if anything breeds inside the sealed storage accumulators. The final item inside the house water system, that has proved to be essential, is an in-line Spirovent de-aerator. This removes all the very tiny micro bubbles of air/ozone that remain in the water, and stops the water coming out of the taps looking like milk. It also stops these tiny air bubbles from interfering with the ultrasonic flow meter in the Sunamp PV, which is primarily why I fitted it.

The water quality from this system is excellent. The water going in is very smelly (from the hydrogen sulphide) and contains 50% more iron that is allowable. The water that comes out has been tested and has no significant contaminants at all, in fact the lab reckoned it was better that the water from most of the water companies in terms of the very low levels of metals and sulphides, particularly. It also had no detectable bacteria, which isn’t really that surprising given the double disinfection used. I could probably take the UV unit out, as I’m sure that the ozone is more than enough to keep the water safe, but I’d already fitted it before I added the ozone treatment, so it’s easier to leave it in than to take it out.

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Stones

posted July 2 2016

Absolutely fascinating.

It’s very easy to see why people struggle to get borehole supplies up and running satisfactorily given the effort you have expended in resolving all of the issues you faced. Are you able to put a figure on how much you have saved by doing this all yourself rather than employing a specialist company to do everything on your behalf?

What are your ongoing service and maintenance costs likely to be, as I assume you will need to keep on top of it / monitor what’s going on?

The big question, how would you rate the treated water from your borehole vs mains water as a drink in its own right, and for making a cuppa?

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PeterStarck

Posted July 3 2016

Amazing! That’s a lot of detail. I’m glad I can use mains supply. I’ll have to get my borehole working now so I can have my water analysed just out of interest.

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JSHarris

Posted July 3 2016

On 02/07/2016 at 23:18, Stones said:

Absolutely fascinating.

It’s very easy to see why people struggle to get borehole supplies up and running satisfactorily given the effort you have expended in resolving all of the issues you faced. Are you able to put a figure on how much you have saved by doing this all yourself rather than employing a specialist company to do everything on your behalf?

What are your ongoing service and maintenance costs likely to be, as I assume you will need to keep on top of it / monitor what’s going on?

The big question, how would you rate the treated water from your borehole vs mains wateronas a drink in its own right, and for making a cuppa?

Most of my effort was a direct consequence of the lack of knowledge in the UK. Had one of the two US guys I was emailing been over here he’d have had a system installed and running perfectly in a day, using off-the-shelf parts, as soon as he had dipped a couple of test papers in the raw water and seen what needed doing to it. He wouldn’t have added the ozone, because really that’s a bit of over-kill on my part, but would have used an off-the-shelf Clack or Microniser air injector in the pump line and just let it pull surrounding air in when the pump was running.

The problem for me was that, AFAIK, no one stocks any of this stuff in the UK. The price of the Clack automatic flow control air injector is $250, plus shipping, plus import duty and tax, for a bit of machined PVC with a flow control washer and one of their venturi injectors. The cost to me of making the same part was around £10 in plumbing fittings and an offcut of brass bar, around a fiver for the Clack venturi and around £30 to import the Dole Valve from the USA (I bought it on Ebay). I paid around £8 for the special non return valve, so all together the home made injector system cost a bit under £55, and I have the advantage of being able to replace any component cheaply if need be. The imported valve would cost around £250 to £300 by the time I’d paid the shipping, import tax and duty, I suspect.

The reaction tank, complete with head unit, riser tube and 3/4″ BSPF ports, cost around £130, delivered. It’s a spare for a water softener, or deioniser used by window cleaners, so there are several sources for them, but I’ve found GAPS to be the most reliable.

Had I realised that aeration was the answer, I’d not have chosen the Aquamandix and sand filter I have, but would have gone for a smaller unit, using Turbidex filtration media. The filter I have cost around £650 delivered (inc VAT) but if I was doing it again I’d go for a smaller Turbidex filter at around £450 inc VAT, which has a higher max flow rate and far better filtration (1500 litres per hour versus 900 litres per hour for the filter I have). It also uses a lot less backwash water, as Turbidex is easier to clean.

I can’t be sure of the costs, but I was quoted £2,800 plus VAT for a basic backwash filter installation, and frankly there’s less than a day’s work in putting the whole system together, even if you have to run the backwash drain around 20m, as I did, to get it to a soakaway.

Maintenance costs come down to replacing the UV tube every year (around £30) and rinsing out the 5µ filter at the same time. A replacement filter when it can no longer be washed clean is around £10. The rest of the kit has no maintenance, just checks to make sure nothing has broken or failed. I have spares for the ozone generator, as it made sense to buy a couple, rather than just one.

The treated water tastes a lot better than tap water here, and makes much better tea. Much of that is down to it being a lot softer and not having any chlorine in it. It has no smell or taste all, whereas our water at home always has a slight smell of chlorine as it comes out of the tap.

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Inchbyinch

Posted July 3 2016

Phenomenal detail as ever Jeremy. Did you talk to any pump and well suppliers from this side of the water. Well drilling is very common here but that said all the equipment come from the states.

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Nickfromwales

Posted July 3 2016

After following your borehole thread ( equally fascinating ), I’d like to say thanks again for the incredible detail and effort in that post 🙂

The Venturi was the bit I couldn’t grasp, but now it’s crystal clear.

( Also good to see some old school soldered copper 😉).

The only bit that’s left me scratching my head is the recirculating circuit via the Dole valve. Surely that re-introduces already ozone ‘airated’ water back into the ozone injection tee inlet, so in essence the water is getting a double dose, or an infinitely increasing dose of ozone depending on how much it recirculates?

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JSHarris

Posted July 3 2016

Yes, I did talk to a lot of drilling companies, but here they seem to specialise in just drilling the hole and dropping the liner and pump in, then leaving the water treatment, if needed, to another company. It seems the majority of boreholes are drilled for farms or industrial purposes, where there may not be a need for treatment. On my mother’s farm the (very old – probably Victorian) borehole produces iron-rich water (it’s in Cornwall, where all the ground water is acidic and often rich in a cocktail of dissolved metal compounds). When that was in use the water was pumped to a large brick-built above ground water storage cistern, standing high in one corner of the farm yard (it’s still there). This cistern had an open top, with just a ventilated cover to keep animals out, and used to naturally oxidise the water over a period of days. When I last looked in it there was a thick layer of orange ferric oxide on the bottom. My guess is that when it was in use it would have been periodically emptied and cleaned out with a shovel. Ferric oxide is dense, so settles quickly in water.

I also spoke to a few water treatment companies. Most were unhelpful, with the sole exception of GAPS Water Treatment. They were very helpful, but being in Rochdale meant they weren’t really able to come down and advise directly. The local advice I did get was deeply flawed or it was clear that the company just didn’t understand water chemistry. The majority of boreholes used for water supplies around the south and south east of England are into chalk, where the water is pretty much free from all contaminants except calcium and magnesium carbonates, from the chalk. The only treatment needed is usually a water softener and perhaps a UV unit to be safe.

I found no one with knowledge of iron removal, and this was supported by the view of the Environmental Health Officer who came to sample our water for official public health analysis in April. She was very interested in the iron removal system, and said that they frequently had to condemn drinking water from around our area because of the high iron content, and consumers were always asking for advice on how to reduce it, and she didn’t know of any local companies that could help. She now has my contact details, as I’ve offered to give advice, as the system I’ve built reduces the iron in the water from 480 ppm as it comes out of the borehole, to less than 10 ppm (the minimum resolution of the lab test – it’s probably zero!) after treatment. The recommended safe limit is 200 ppm of iron in drinking water.

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JSHarris

Posted July 3 2016

On 03/07/2016 at 10:47, Nickfromwales said:

After following your borehole thread ( equally fascinating ), I’d like to say thanks again for the incredible detail and effort in that post 🙂

The Venturi was the bit I couldn’t grasp, but now it’s crystal clear.

( Also good to see some old school soldered copper 😉).

The only bit that’s left me scratching my head is the recirculating circuit via the Dole valve. Surely that re-introduces already ozone ‘airated’ water back into the ozone injection tee inlet, so in essence the water is getting a double dose, or an infinitely increasing dose of ozone depending on how much it recirculates?

Thanks Nick.

Nothing recirculates because there is a big pressure differential whenever the borehole pump and the ozone generator are running. With the pump running the area below the Dole Valve and the injector venturi is sitting at around 10 bar, whereas the area above them is at between 3.4 and 4.5 bar, depending on how full the accumulator tanks are downstream. This means that water is always flowing up the pipe above the venturi into the lower branch of the top tee, and water at the same pressure is also flowing in to the side of that same tee. Arranging it like this ensures good mixing at the tee, as there will naturally be a lot of turbulence created by the incoming water from the Dole Valve inlet at the side, just because it’s at 90 deg to the flow.

What’s not shown is that the riser pipe connects to a length of 25mm PVC pressure pipe, with what was a clear section (this is 16 bar industrial water pressure pipe with bonded joints, not waste pipe!). The idea of the larger bore pipe was two fold, to get some expansion that would reduce the local pressure and aid aeration and to allow me to see whether the aeration system was working, by looking for loads of bubbles in the water (it did this very well for a few days, until the inside of the clear pipe went brown from the rust sticking to it…….). This is what it looks like, with another view of the annotated photo with a line showing the pressures in each half:

The other PVC downpipe, the large bore flexi and the sloping pipe with the gate NRV, is the waste drain from the backwashing filter, plus a manual emptying ball valve for the aeration tank, so I can empty it out for servicing (it siphons itself empty when the ball valve is opened, as the outlet pipe goes right to the bottom of the tank).

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Nickfromwales

posted July 3 2016

So the Dole valve is there ‘in case’ the differential changes? Eg water isn’t always flowing through it? I just thought the ozone T would have been after the recirculating bypass created by the DV not before it. Got me thinking on a Sunday morning anyway 🙂

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JSHarris

Posted July 3 2016

No, the Dole Valve creates the pressure differential across the venturi, by restricting the volume that would flow if it wasn’t there and was just an open pipe. The pump will easily deliver 30 litres per minute or more, so the Dole Valve restricts the volume flow when the pump is running to a set flow rate. The attached pdf shows how it works, but it’s very similar to the flow restrictors fitted to showers and taps to reduce flow rate, just better engineered and a lot more robust (in fact, fairly typically American!).

The Dole Valve allows a constant 9.5 litres per minute (really 2.5 US gallons per minute) to flow around the bypass loop as long as there is a pressure differential across it of between 15psi and 125psi (1.03 to 8.62 bar). In practice the lowest pressure across it is when the pump is just about to turn off, which is about 5.5 bar (10 bar – 4.5 bar turn off pressure) and the highest pressure across it is when the pump first turns on, at the lower limit of 3.4 bar, and is about 6.6 bar (10 – 3.4 bar turn on pressure), so it regulates flow pretty accurately.

Without the Dole Valve loop the pressure differential would be very high across the venturi (the pump stalls at around 20 bar, IIRC) and would be over the 16 bar working pressure rating of the pipe or the 12 bar working pressure rating of the fittings. The flow into the tank would be tiny, about 1 litre per minute (the venturi will flow at a higher rate when the pressure is higher, because it has a fixed nozzle diameter).